The present invention relates to carbon technology, and more particularly to highly activated carbons and the use of such carbons in cigarette filters to selectively oxidize or adsorb selected gas phase components.
Certain activated carbons offered commercially show xe2x80x9cintrinsic catalytic activityxe2x80x9d for oxidation of toxic gaseous molecules such as phosphine, hydrogen sulfide, arsine or sulfur dioxide, to products which form a condensed phase that remains adhering to the carbon. Such carbons include Calgon Carbon Corporation""s CENTAUR(copyright) carbon (see U.S Pat. No. 5,356,849, granted Oct. 18, 1994; U.S. Pat. No. 5,444,031, granted Aug. 22, 1995; U.S. Pat. No. 5,464,605, granted Nov. 7, 1995; and U.S. Pat. No. 5,674,462, granted Oct. 7, 1997; all incorporated herein by reference), and NORIT Americas"" Vapure 610, 612 or 610P line of carbons. These carbons are both made from coal but by very different processes. CENTAUR(copyright) specifically attempts to incorporate nitrogen into the structure by high-temperature treatment with nitrogenous substances such as ammonia or urea. NORIT Vapure has no such explicit nitrogen thermal treatments; however, the intrinsic nitrogen content of the source coal may be sufficient to leave enough in the final product, given an appropriate set of manufacturing conditions. One feature that both carbons have in common that sets them apart from carbons made from coconut shells or wood (for example), is a substantial content of metals (often referred to by the catch-all phrase xe2x80x9cashxe2x80x9d). Carbon manufacturers have a significant under-appreciation of the importance of the specific ash components to catalytic activity. Moreover, such producers of activated carbons have a limited understanding of the precise mechanistic details underlying xe2x80x9cintrinsic catalytic activityxe2x80x9d of carbons so favored with such properties. Of the various metals found by PIXE analysis to occur in either carbon to extents of hundreds or thousands of parts per million, the one that dominates both is iron. Iron content of either CENTAUR(copyright) or NORIT Vapure typically ranges from 0.2 to 0.4% metal by weight.
In accordance with the present invention, catalytically active carbon impregnated with heavy metal is used as a filter element for cigarettes and the selective adsorption and/or oxidation of selected gas phase components in cigarette smoke. The carbons are impregnated with heavy metals and sulfur or nitrogen ligation, and the activity of such carbons is significantly improved by introducing catalytically active sites by heat treatments in the range of 500-1000xc2x0 C. in the presence of heavy metals and nitrogenous materials.